The present invention relates to the transmission of a reference signal necessary for the mutual synchronization of telecommunications devices in a transmission network.
For any synchronous transmission between two telecommunications devices, the devices involved must always be synchronized. Among other things, synchronization is required for compensation of frequency errors between the devices. Typically, synchronization is carried out using a reference signal relayed to all the devices involved in transmission. Terminal devices may receive this reference signal from the transmission network they are using. Alternatively, one of the devices involved transmits a reference signal that the other devices can then use for synchronization.
An example of an arrangement where one of the two transmission devices in a system synchronizes with another device by means of a reference signal transmitted by the former is transmission between a Base Transceiver Station BTS and its distributed Antenna Unit AU in a telecommunications network. Traditionally, such transmission is carried out by means of special cabling laid between the Base Transceiver Station and the Antenna Unit. However, if one Base Transceiver Station is to be connected to several distributed Antenna Units, the cost of cabling may prove prohibitive.
Data can also be transmitted between the Base Transceiver. Station and its Antenna Unit via a separate, already existing transmission network. An example of such a transmission network that is readily available is the cable television network. At present, the cable television network is fairly extensive and offers unused capacity, making it possible to use it for the transmission of data other than just the television signal. As it is, in addition to television signals, the cable television network is also being used, for example, for transmitting other data using the so-called cable modems.
FIG. 1 shows an example of a system where the signal between the base transceiver station BTS 101 and the antenna unit AU consisting of the transmitter TX 105 and receiver 111 is transmitted via a separate transmission network. The transceiver unit (not shown) is connected to an antenna, by means of which the mobile stations can communicate with the base transceiver station and make use of -the services offered by the network. For the sake of clarity, the figure only shows one antenna unit AU, but the same base transceiver station can communicate simultaneously with several transceivers. In this example, the transmission network is a cable television network, where the signal is transmitted following conversion for the frequency of the television channel used for transmission.
The modulated signal with a bandwidth of 200 kHz on the 1930-1990 MHz band in an air interface traffic channel conforming to the mobile communications system specifications and which is to be transmitted from the base transceiver station to the transmitter TX of the antenna unit AU is converted in the adapting unit 102 for the free channel on the band used for the connection in a transmission network. For the connection, the band 180-810 MHz may be selected, with the exception for certain narrow bands that are disallowed. At the receiving end, the signal is converted by the adapting unit 104 into its original band of 1930-1990 MHz.
The signal in the 1850-1910 MHz band to be transmitted by the antenna unit receiver RX to the base transceiver station BTS is converted in the adapting unit 112 for the band 10-105 MHz to be used for the connection in the transmission network. At the receiving end, the signal is re-converted in the adapting unit 114 into its original band 1850-1910 MHz. In addition to the traffic channel signal, the reference signal SREF required for the synchronization of the terminal devices must also be transmitted over the transmission network.
The frequency ranges to be used are not critical to the use of the system, and so the GMS-1900 frequencies used in this example can be replaced by GSM frequencies, in which case the signal from the base transceiver station to the transmitter TX is in the 935-960 MHz band and the signal from the receiver-to the base transceiver station in the 890-915 MHz band, or DCS-1800 frequencies, in which case the signal from the base transceiver station to the transmitter TX is in the 1805-1880 MHz band and the signal from the receiver to the base transceiver station in the 1710-1785 MHz band.
The operation of the transmitting-end adapting units 102 and 112 is discussed below.
Generally, the task of the adapting units is to convert the signal modulated to the frequency f1 into such a form that the signal can be transmitted over a channel operating in the frequency f. Let us first examine the processing of just the modulated signal to be transmitted over a traffic channel. The operation of the adapting unit 102 in FIG. 1 is illustrated in FIG. 2. Except for the frequency bands to be used, the adapting unit 112 operates in an identical manner. The unit input consists of the modulated downlink signal S201 in the band 1930-1990 MHz obtained by modulating the signal s(t) to be transmitted by the base transceiver station over the traffic channel. In the adapting unit 102, the signal S201 is first converted in the mixer 202 by means of the mixing signal SMIX having another frequency of fMIX. Thus, the signal S202 is obtained in addition to the input signals,
S202=S201xc2x7SMIX,xe2x80x83xe2x80x83(1)
where two frequencies are present, fTCH+fMIX and fTCHxe2x88x92fMIX (fTCH being the frequency of the modulated traffic channel signal S201 and fMIX the frequency of the mixing signal SMIX). fMIX is selected so that one of the two frequency components of the mixed signal is consistent with the frequency channel of the transmission network to be used (such as a cable television network). If, for example, a modulated traffic channel signal with a frequency of 1951 MHz is to be adapted for transmission in a cable television network at a frequency of 546 MHz, the mixing frequency fMIX of 1405 MHz will be selected. At the same time, the other signal components remain outside the frequency range of the channel reserved for transmission, and so they must be filtered. This is achieved by means of the band-pass filter 203, which filters the signal components that fall outside the frequency range of the channel operating at the frequency f (e.g. 545 MHz less than f less than 555 MHz). The signal S203 obtained from the band-pass filter is fed into the transmission network.
The signal s(t) travels in the transmission network in its original bandwidth but at a new center frequency f=fTCHxe2x88x92fMIX=546 MHz. The signal S203 conforms to the band reserved in the cable television network for the connection between the transceiver station BTS and transmitter TX and can be fed into the cable television network as it is.
At the receiving end, the modulated traffic channel signal S201 sent from the base transceiver station BTS to the transmitter TX and converted by the adapting unit 102 will be reconstructed by the adapting unit 104, FIG. 1, whose operation is explained in FIG. 3. Similarly, the signal from the receiver RX to the base transceiver station BTS converted- by adapting unit 112 is reconstructed by unit 114 in exactly the same way, except for the frequency bands. Several signals are being transmitted in the transmission network (such as a cable television network) simultaneously over several channels, the individual signals constituting the total signal S. The channel for the transmission connection is selected by suppressing the other signals by means of the band-pass filter 301. The signal S301 with a center frequency of fTCHxe2x88x92fMIX=546 MHz obtained must be re-converted for its original frequency band of fTCH. To accomplish this, the signal is mixed in the mixer 302 using the mixing signal Sxe2x80x2MIX with a frequency of fMIX generated by the signal generator 312 to produce the signal S302.
Following processing by the mixer 302, the signal includes, in addition to the input signals, two new frequency components, namely the original frequency fTCH≈1951 MHz and the second side band |fTCHxe2x88x922fMIX|≈860 MHz obtained as a result of mixing. Any superfluous and interfering frequency components are eliminated by the band-pass filter 303, and the required component S303 with the frequency fTCH is amplified by the amplifier 304 to produce the signal S304 which is a copy of the original signal S201 shown in FIG. 2 and transmitted from the transmitting end.
The mixing signals SMIX and Sxe2x80x2MIX in FIGS. 2 and 3 must have exactly the same frequency because a difference in frequency will produce a frequency error in the signal S304 to be transmitted over the transmission network, the said error being equivalent to the difference in frequency. To ensure that the signal S304 to be transmitted via the radio path has the a sufficiently accurate frequency and to enable the transmitting and receiving ends to communicate, the transmitting and receiving ends must be synchronized. For example, the GSM specifications require a frequency accuracy of xc2x10,2 ppm, which means that the equipment must be accurately synchronized and remain so. Synchronization can be achieved by means of sufficiently accurate clocks at both the receiving and transmitting ends. However, clocks of such accuracy are too expensive for commercial applications. Typically, a common reference signal is used for transmission between devices.
Let us examine a situation where a reference signal in a system such as illustrated in FIG. 1 is transmitted from the base transceiver station BTS to the adapting units 102, 104, 112 and 114, to the transmitter 115 and the receiver 111. Thus, the adapting unit 102 must transmit both the downlink traffic channel signal and the reference signal intended for the receiving end. The adapting unit 112 does not have to transmit any reference signal. Otherwise in terms of operation, the unit 112 is identical with the unit 102 except for the frequency bands used, so that an analysis of the operation of the unit 102 is sufficient in this context. The operation of the units 102 and 104 in a system where the reference signal is transmitted over the transmission network together with the traffic channel signal is illustrated in FIGS. 4 and 5.
FIG. 4 illustrates the operation of the unit 102, shown in FIG. 1, that transmits both the traffic channel signal S401 and the reference signal SREF over the transmission network. Typically, the reference signal SREF is transmitted on a different frequency band than the traffic channel signal S401. The following examples discuss a case where the frequency of the reference signal SREF is fREF=13 MHz. When the reference signal SREF is added to the signal S401 in FIG. 4, which has a frequency of fREF=1951 MHz, the total signal S402=S401+SREF is obtained for transmission to the mixer 402. The total signal includes two frequency components, fTCH and fREF.
When the total signal S402 is modified by the mixer 402 with the mixing signal with a frequency of fMIX=1405 MHz, the signal S403 is obtained. The signal S403 includes four new frequency components, with the first two corresponding to the modulated traffic channel signal S401 being converted into the center frequencies of fTCH+fMIX=3370 MHz and fTCHxe2x88x92fMIX=546 MHz, and the other two to reference signal SREF being converted into the frequencies fREF+fMIX32 1423 MHz and fREFxe2x88x92fMIX=1397 MHz. The band-pass filter 403 suppresses the components that do not fit in the frequency channel 545 MHz less than f less than 555 MHz. The band-pass filter provides the signal S404 to be transmitted to the transmission-network. Of the original signal S403, the only component left is fTCHxe2x88x92fMIX, which operates at a frequency of 546 MHz.
FIG. 5 shows the adapting unit 104 at the receiving end, where the signal is re-converted into its original frequency of fTCH. The transmission channel used is selected with the band-pass filter 501. The signal obtained, which is identical with the signal S404 in FIG. 2, is mixed by the mixer 502 using the mixing signal Sxe2x80x2MIX. To ensure that the signal S503 produced by the mixer is an exact copy of the signal S402 in FIG. 4, the frequency of the mixing signal Sxe2x80x2MIX must be exactly the same as that of the mixing signal SMIX in FIG. 4. Therefore, SMIX must be generated using the reference signal SREF to be transmitted over the transmission network. An attempt is made to regenerate the reference-signal from the signal S503 by means of the band-pass filter 521. However, the synchronization signal SREF has been suppressed by the filter 403 in FIG. 4, so that S503 does not include the necessary reference signal, and the signal SREF2 obtained form the band-pass filter 521 is just noise. As a result, the phase-locked loop 511 does not receive its reference signal SREF, and reconstruction of S401 fails.
Consequently, it is the transmission of the reference signal that poses the problem in transmission. Because it is not possible to feed a signal external to the channel to the transmission system, the reference signal is filtered out by the band-pass filter. The idea of the present invention is to eliminate or at least alleviate the shortcomings of the state of the art. This objective is achieved by means of the method described in the enclosed independent patent claim.
The idea of the invention is to convert the reference signal, prior to sending it to the transmission network, into a frequency that can be transmitted via the channel used by the transmission devices connected to the transmission network. The reference signal can be transmitted, for example, on a free channel reserved for a data signal or another channel specifically reserved for the transmission of the reference signal. At the receiving end, the reference signal is reconverted to its original frequency.
A reference signal transmitted together with the data signal via the same transmission path is very much subject to the same delays and non-ideal conditions as the data signal. Similarly, the phase and frequency differences of the reference and data signal remain unchanged in transmission. As a result, the transmitted data signal can be accurately reconstructed at the receiving end from the data signal converted for the transmission network channel involved.
In a preferred embodiment of the invention, the signal from the base transceiver station to the antenna unit is constructed in the form of a television signal. Then, the reference signal can be converted at the receiving end for a television voice channel, which makes it easy to deconstruct it at the receiving end by means of the voice circuits known from televisions.